Recently, demand has increased for housing materials with high technology and multifunctional properties as large and light-weight products are becoming more widespread. In particular, as the appearance of molded products has become important, there is a need for a high glossy material which can provide both scratch resistance and high grade texture, in addition to a flame retardant material having stability against fire.
One method for obtaining both scratch resistance and flame retardancy is to alloy polycarbonate (PC) resin and methacrylate resin, such as polymethylmethacrylate (PMMA). The polycarbonate resin has good mechanical strength, transparency, thermal stability, self-extinguishing property, and dimensional stability so that it has been widely used in electrical and electronic products and parts of automobiles. Further, the polycarbonate resin can obtain flame retardancy even with a small amount of flame retardant, compared to a conventional polymer, since its chemical structure allows good flame retardancy. However, polycarbonate resin does not by itself exhibit good scratch resistance because the polycarbonate resin has a low pencil hardness of B or so. On the other hand, although PMMA has a high pencil hardness of 3H˜4H, and thus can exhibit good scratch resistance, it can be difficult to impart flame retardancy to the PMMA resin with a conventional flame retardant. Accordingly, blending a PC resin and a PMMA resin has been suggested to improve both scratch resistance and flame retardancy.
However, even when the PC resin and the PMMA resin are melt-blended at a high temperature, they may divide into separate phases due to their lack of compatibility. It can also be very difficult to use an alloy of PC and PMMA in housings of electrical and electronic products. Due to the different refractive indices of the PC resin and the PMMA resin (1.59 and 1.49, respectively), the alloy of PC resin and PMMA resin may scatter light. This makes it difficult to provide color with high chroma, and a melt joining line is clearly shown during extrusion.
Korean Patent Publication Laid-open No. 2004-79118 discloses a method of lowering the molecular weight of polycarbonate during a melt blending process using metal stearic acid ester in order to improve the compatibility between a polycarbonate resin and a methacrylate resin. However, the method has a disadvantage in that a blend of the polycarbonate and the methacrylate resin has limited transparency and significantly low mechanical properties.
U.S. Pat. No. 4,027,073 discloses a method of coating the surface of a resin to improve scratch resistance. The method, however, requires an additional process step.